Transaction history in LLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a transaction history in system design?

Transaction history is a record of all actions or changes made to data over time, allowing tracking and auditing of past operations.

intermediate

Why is immutability important in transaction history design?

Immutability ensures that once a transaction is recorded, it cannot be changed or deleted, preserving data integrity and auditability.

beginner

Name two common storage options for transaction history data.

1. Append-only logs or event stores 2. Relational databases with audit tables

intermediate

What is the role of indexing in transaction history systems?

Indexing helps quickly retrieve transaction records by attributes like user ID, date, or transaction type, improving query performance.

beginner

How does pagination help in displaying transaction history?

Pagination breaks large transaction lists into smaller pages, making it easier to load and view data without overwhelming the system or user.

What is the main purpose of maintaining a transaction history?

A To track and audit all changes over time

B To delete old data automatically

C To speed up data entry

D To encrypt user passwords

Transaction history records all changes to enable tracking and auditing.

Which storage type is best suited for immutable transaction records?

A Session storage

B Temporary cache

C In-memory database

D Append-only log

Append-only logs store immutable records by only adding new entries.

Why is indexing important in transaction history systems?

A To encrypt data

B To improve query speed

C To delete old transactions

D To compress data

Indexing speeds up searching and retrieving transaction records.

What does pagination help with when showing transaction history?

A Breaking data into manageable pages

B Encrypting transaction data

C Deleting duplicate records

D Backing up data

Pagination divides large data sets into smaller pages for easier viewing.

Which property ensures transaction history cannot be altered after recording?

A Compression

B Encryption

C Immutability

D Caching

Immutability means data cannot be changed once recorded.

Explain the key design considerations when building a transaction history system.

Think about how to keep records safe, easy to find, and easy to view.

Describe how transaction history supports auditing and troubleshooting in applications.

Consider why knowing past actions is important for fixing problems and following rules.

Practice

1. What is the main purpose of a transaction history in a system?

easy

A. To record all important actions with details for tracking

B. To speed up the system by caching data

C. To delete old data automatically

D. To encrypt user passwords

Solution

1. Step 1: Understand the role of transaction history

   Transaction history stores records of actions with details like timestamps and IDs.

2. Step 2: Identify the correct purpose

   This helps users and systems track past events clearly and reliably.

3. Final Answer:

   To record all important actions with details for tracking -> Option A

4. Quick Check:

   Transaction history purpose = record actions [OK]

Hint: Transaction history = record actions with details [OK]

Common Mistakes:

• Confusing transaction history with caching
• Thinking it deletes data automatically
• Mixing it with security features like encryption

2. Which of the following is the correct way to uniquely identify each transaction in a history system?

easy

A. Using a timestamp only

B. Using a unique transaction ID

C. Using the user's name

D. Using the transaction amount

Solution

1. Step 1: Identify unique identifiers in transaction history

   Unique transaction IDs ensure each record is distinct and traceable.

2. Step 2: Compare options

   Timestamps alone can repeat; user names and amounts are not unique identifiers.

3. Final Answer:

   Using a unique transaction ID -> Option B

4. Quick Check:

   Unique ID = unique transaction record [OK]

Hint: Unique transaction ID ensures distinct records [OK]

Common Mistakes:

• Assuming timestamp alone is unique
• Using user name as unique key
• Using transaction amount as identifier

3. Given this simplified transaction record list:

transactions = [
  {"id": "t1", "time": "2024-01-01T10:00:00Z"},
  {"id": "t2", "time": "2024-01-01T09:00:00Z"},
  {"id": "t3", "time": "2024-01-01T11:00:00Z"}
]

What is the correct order of transaction IDs if sorted by time ascending?

medium

A. ["t1", "t2", "t3"]

B. ["t2", "t3", "t1"]

C. ["t3", "t1", "t2"]

D. ["t2", "t1", "t3"]

Solution

1. Step 1: Analyze timestamps for each transaction

   t2 = 09:00, t1 = 10:00, t3 = 11:00 in UTC time.

2. Step 2: Sort transactions by ascending time

   Order is t2 (earliest), then t1, then t3 (latest).

3. Final Answer:

   ["t2", "t1", "t3"] -> Option D

4. Quick Check:

   Sorted by time ascending = [t2, t1, t3] [OK]

Hint: Sort by timestamp ascending for correct order [OK]

Common Mistakes:

• Sorting by ID instead of time
• Confusing ascending with descending order
• Ignoring timestamp format

4. You have this code snippet to add a transaction record:

def add_transaction(history, transaction):
    if transaction['id'] not in [t['id'] for t in history]:
        history.append(transaction)
    else:
        print("Duplicate transaction")

history = [{"id": "t1"}]
add_transaction(history, {"id": "t1"})

What is the output when running this code?

medium

A. Duplicate transaction

B. KeyError exception

C. No output, transaction added

D. TypeError exception

Solution

1. Step 1: Check if transaction ID exists in history

   The code checks if 't1' is already in the list of IDs in history.

2. Step 2: Since 't1' exists, print duplicate message

   The else branch runs and prints "Duplicate transaction".

3. Final Answer:

   Duplicate transaction -> Option A

4. Quick Check:

   Duplicate ID detected = print message [OK]

Hint: Check for existing ID before adding to avoid duplicates [OK]

Common Mistakes:

• Assuming transaction is added anyway
• Expecting an exception instead of print
• Confusing list comprehension syntax

5. You want to design a scalable transaction history system for millions of users. Which approach best ensures fast retrieval of a user's transactions sorted by time?

hard

A. Store transactions in separate files per day without indexing

B. Store all transactions in one big list and scan it every time

C. Use a database with an index on user ID and timestamp

D. Keep transactions only in memory without persistence

Solution

1. Step 1: Consider scalability and retrieval speed

   Scanning one big list or files without index is slow for millions of users.

2. Step 2: Use database indexing on user ID and timestamp

   This allows fast queries to get transactions per user sorted by time efficiently.

3. Step 3: Avoid in-memory only storage for persistence and scale

   Memory-only storage risks data loss and limits scale.

4. Final Answer:

   Use a database with an index on user ID and timestamp -> Option C

5. Quick Check:

   Indexing = fast retrieval at scale [OK]

Hint: Index on user ID and timestamp for fast queries [OK]

Common Mistakes:

• Scanning large lists for each query
• Ignoring indexing benefits
• Relying on memory-only storage